Transmitted-light illuminating devices for microscopes in the higher-price category have a centerable and focusable condenser, an iris-aperture diaphragm in the rear focal plane of the condenser, as well as an indexible turret. The turret contains centerable diaphragms (or auxiliary lenses), polarizers and filters for performing different contrasting methods, as well as selectively attachable auxiliary lenses for illuminating large fields of view or for magnifying enlargement. Since a large number of adjusting means must be actuated, such illuminating devices are not easy to handle and they entail the danger of erroneous operation even for the expert, as for example, by mistakenly closing the aperture diaphragm when making a phase-contrast observation.
To avoid such operating error, West German published patent application OS No. 3,042,186 discloses a coupling of the iris-aperture diaphragm to the turret via a spring and a control lever, in such manner that the diaphragm remains open in the "phase contrast" position of the turret. But this known illuminating device achieves no reduction in the required number of adjustment levers; the iris-aperture diaphragm and the turret are actuated by separate setting levers.
Furthermore, illuminating devices for less expensive microscopes, as for school instruction, are known to be of greatly simplified construction However, their simplification is obtained at the expense of important functions, as for example, by omitting any provision for vertical displacement of the condenser, thus precluding exact setting of Kohler illuminating conditions.
In one such illuminating device having a fixed condenser, it is known to couple the actuating lever of a switchable auxiliary lens or ring diaphragm, to the control member for the aperture diaphragm. This known device comprises a switch rocker which is fastened on a pivot shaft which can be displaced about the optical axis. When the rocker is reversed, the lens holder engages in a slide seat on the condenser housing; it can be rotated about the optical axis, via the actuating lever of the rocker, driving along with it the finger which controls the diaphragm.
In this device, therefore, the diaphragm-actuating lever is eliminated, without dispensing with the function of the iris diaphragm, while at the same time the iris-aperture diaphragm is opened fully upon switching into the "auxiliary lens" or "phase-contrast" position; and upon switching back into the starting position, the diaphragm is also closed again.
But this known device has the disadvantage that the number of switch positions is limited to two. There are no additional possibilities of contrasting, aside from the bright-field illumination of small-object fields and, upon addition of a lens, of large-object fields. If it is now desired to use the microscope for phase contrast, the entire switch rocker must be replaced by one which carries, in place of the auxiliary lens, a ring diaphragm which can be centered by means of knurled screws.
It is not readily possible to avoid this troublesome replacement by increasing the number of switch positions, since the range of movement of the switch rocker is limited, by reason of its engagement with the control finger of the diaphragm.